تأثیر موسیلاژ دانه‌ شاهی بر پایداری و خواص رئولوژیکی دوغ بدون چربی

Introduction: Doogh is a native beverage in Iran and has an import share in beverage industry. Also quality improvement is needs for this native industrialized product. Rheological Behavior in macro and micro scale are the most interesting properties affecting the quality of the food (kiani and et al 2008). The rheological characteristics indicate a possible use as a novel thickener. The dynamic viscosity of the dilute solution shows liquid behavior, and at higher concentrations, viscoelastic and gel behaviors (Razavi and et al 2014). Flow properties of food are determined for a number of purposes, such as quality control, understanding the structure, process engineering application and correlations with sensory evaluation. Also final properties of the product such as stability, texture and appearance are directly depending on food microstructure (kiani and et al 2008). The major problem with this product is phase separation during storage, due to the low pH and aggregation of casein may be seen frequently in   doogh (soltani   and et al 2012). Hydrocolloids have been widely used in textural stabilization of fermented milk products (Koksoy and   Kilic 2004). Gums from seeds are now becoming important additives in the food industry. According to the increasing demand for hydrocolloids, with specific functionality in the recent years, finding new hydrocolloid sources with appropriate properties is an active area of study. Among commercial gums, the hydrocolloids from seeds are still used extremely in food formulations because of their appropriate price, easy availability and proper functionality (Naji-Tabasi and et al 2011). Cress seed gum exists in the envelope alongside the outer layer of the seeds of garden cress (Lepidium sativum L.) plant The seeds consist of 6.5–15% of the mucilage, which contains cellulose (18.3%) and uronic acid containing polysaccharides. Cress seed gum offers a wide variety of food applications because of its yield stress, pseudoplasticity and thixotropy behavior. It is stable in salts (NaCl and CaCl2) and over a wide range of pH’s, with a synergic effect of added sugars (sucrose and lactose) on its flow behavior. It offers applications in emulsifying, emulsion stabilization and foaming properties. It is also stable on heating, cooling and freezing (Razavi and et al 2014).  In this study effect of cress seed mucilage was investigated as a native gum and doogh stabilizer during 28-days storage period at a refrigerated temperature of 5ºC.
Material and methods: In this experiment, Cress seed and yogurt (0.5% fat with acidity 134°D) were bought from herbal medicinal shop and dairy products company (Laleh Sari –Iran) respectively. Then, extracts of dry cress seeds were prepared. Briefly, cress seed was soaked in preheated de-ionized water at a water/seed ratio of 30:1. 0.1 mol /L NaOH solution was used to adjust the pH to 10. The slurry was stirred continuously for about 15 minutes in constant temperature (35˚C). An extractor with a rotating rough plate was employed to cut the gum layer off the seed. This degummed seeds were discarded; finally, the slurry was dried with the 60˚C air forced oven and milled to powders. They were kept in cool and dry condition (Behnia and et al 2013). The gum was then randomly assigned to treatment groups (0, 0.01, 0.1, 0.3 and 0.5%). Preparation of doogh, after dissolution the mucilage in deionized water (Temperature of 80°C for 10 minutes) by adding 0.7% salt, 40 % yogurt was added to the stabilizer mixture and salt then finally product was homogenized with the Ultratorex Homogenizer (11000 rpm for 30 s in 60^∘C) and doogh without stabilizer was considered as a control sample. Then, viscosity, zeta potential, stability, sensory and chemical properties doogh   samples   were measured.
Results and discussion: The statistical analysis of results showed that the doogh samples with 0.5% stabilizer at a low shear rate had higher viscosity in comparison with the control and other samples (P<0.05) (fig 1), The samples showed a pseudo- plastic behavior by increasing the shear rate (Feyzabadi et al. 2014) and This result was in agreement with the finding of (Razavi and et al 2014); that is the cress seed extract shows a newtonian below 0.1%. However, at highers concentrations there is a pronounced shear thin at higher concentrations there is a pronounced shear thinning behavior in steady shear measurement and viscoelastic and weak gel type behaviors in dynamics tests.  Its viscosity depends on many factors such as shear rate, temperature, time, pH, biopolymer concentration.  The addition of cress seed mucilage had a significant (P<0.05) change on the titratable acidity of doogh samples but pH no changed significant these results maybe because of this IR-FT polymer spectrum of cress seed gum polymer showed that the polymer consists of carbohydrates with carboxyl and hydroxyl functional groups (Karazhiyan and et al. 2010) (Table 1). The doogh samples with 0.3 and 0.5% concentration stabilizer had highest stability during storage time (fig 2).  In other study intuitively the link between high viscosity and good physical stability is obvious and this view suffices for many applications (Syrbe and   et   al. 1998) and Findings from the study by Foroughiniaa et al (2007)., Gum tragacantin was the most effective, resulting in full stability of Doogh at a concentration of about 0.3%.  Based on the sensory evaluation results, the consistency score of the samples increased with increasing gum concentration up to 0.3% compared to the control sample, but the difference in concentration of 0.5% with the control was not significant. Apparently that is usually the absolute value of zeta potential in milk products below 25 mV. Due to the noticeable changes in the zeta potential in the samples, electrostatic repulsion plays an essential and inescapable role in stabilization (Azarikia and et al 2009). The   zeta potential was negative   because of anionic nature of the mucilage   that is - 10.78 ±.19 mv. The higher the numerical value of the zeta potential in the formulation containing gum, the lower the repulsive force between the droplets and their less tendency to stick together, leaving the droplets together to stabilize the system (Mirmajidi et al 2016).
Conclusion: The results showed that the addition of mucilage seeds to the concentration of 0.1% on day 28th during storage caused a significant increase in the stability of the doogh samples (62%) and the concentration of 0.3 and 0.5% resulted in complete stability of the samples. The maximum apparent viscosity in samples containing mucilage seed was 0.5% and then 0.3%. Since the highest consistency rating was observed in the sensory evaluation of the samples containing 3%   cress seed mucilage and also its complete stability compared to the control sample, this   concentration is recommended as the most appropriate level.